1. Field of the Invention
The present invention relates to the field of reception of orthogonal frequency division multiplexed (OFDM) signals. More particularly, the invention concerns demodulation of received OFDM signals and synchronisation at an OFDM receiver.
2. Description of The Prior Art
Orthogonal frequency division multiplexing is a method of transmitting data which is being investigated, because of its good interference properties, for use in the UHF band.
In the discussion that follows it is assumed that an inverse Discrete Fourier Transformation is used to generate the OFDM signal.
In general it will be necessary to modulate a generated OFDM signal onto a carrier so as to reach an appropriate frequency for transmission. By adapting the processing which is performed in the frequency domain it becomes possible to simplify the modulation of the OFDM signal onto a carrier. This is described in our co-pending International patent application No. PCT/GB91/00513.
In order to recover data from a received OFDM signal which has been modulated onto a carrier it is necessary to demodulate the OFDM signal from the carrier onto which it is modulated before demodulating data from the individual OFDM carriers. Embodiments of the present invention may deal with one or both of these types of demodulation.
As with more conventional methods of data transmission, with OFDM signals it is necessary to synchronise the receiver to the transmission before useful data can be recovered. The normal way of achieving this is by using special framing signals, however this represents an overhead on the available bit rate and may compromise the interference properties of the signal.
It has also been proposed, in the article "Digital Implementation of High Speed HF Modems" by D. Harmer and B. Hillam, to synchronise a receiver to the block start positions in a received OFDM signal by integrating each received modulated OFDM carrier and, because there should be an integer number of cycles occurring during one block, a non-zero value for the integral will indicate block misalignment.
Embodiments of the present invention may deal with any or all of block synchronisation, sample clock synchronisation and, where relevant, local oscillator synchronisation, without the need to use special framing signals, by looking at the distribution of the demodulated sample values.
As shown in FIG. 1, an orthogonal frequency division multiplexed (OFDM) signal consists of a large number of carriers each of which is modulated by a signal whose level varies discretely rather than continuously and thus the resulting power spectrum of each carrier follows a (Sin x/x).sup.2 distribution. The symbol rate of the modulating signals, and the carrier frequencies, are such that the peak of power of each modulated carrier occurs at frequencies corresponding to nulls in the power spectrum of the other modulated carriers. The carrier spacing is equal to the reciprocal of the symbol rate of each modulating signal (assuming that all of the modulating signals have the same symbol rate).
The overall spectrum of the OFDM signal is very close to rectangular when a large number of carriers are contained in the OFDM signal.
During a given time period, T, the OFDM signal may be represented by a block of N samples. The value of the kth sample is, as follows: ##EQU1##
The N values X(n) represent the respective values, during period T, of the discretely-varying signals modulating the OFDM carriers e.sup.2jnk/N.
It may be seen from the above equation that the OFDM signal corresponds to the inverse Discrete Fourier Transform of a set of data samples, X(n). Thus, a stream of data may be converted into an OFDM signal by splitting the data stream up into blocks of N samples X(n) and subjecting each block of data samples to an inverse Discrete Fourier Transform.
The succession of data samples, X(n), which appear at a particular sample position over time constitute a discretely-varying signal which modulates a carrier at a frequency, f.sub.n.